Millimeter-wave integrated circuits are being increasingly used for communication and scientific purposes because of their many desirable properties. In these millimeter-wave integrated circuits metallic conductors are established on an insulating substrate by the process of photolithography and semiconductor devices are then mounted on these conductors. One difficulty with the use of millimeter-wave integrated circuits at very short wavelengths is the lack of a satisfactory method of mounting the semiconductor devices on the circuits.
In order to utilize fully the high frequency performance characteristics of millimeter-wave semiconductor diodes these diodes must be mounted in the circuit in a way which introduces a minimum amount of parasitic reactance. The waveguide mount known in the prior art as the Sharpless wafer is one of the closest approximations to a optimum configuration of the device. This type of semiconductor diode is disclosed in the article entitled, "Wafer-Type Millimeter Wave Rectifier" by W. M. Sharpless, Bell System Technical Journal, Vol. 35, 1956, pp. 1385-1402. The Sharpless type of semiconductor diode has a rectifying contact on one surface of a semiconductor chip and an ohmic contact on the opposite surface of the chip. This type of diode chip can be mounted in a waveguide with the ohmic contact attached to one wall of the waveguide and contact is established to the opposite surface of the chip by means of a spring coming from the opposite wall of the waveguide. Because this spring contact is perpendicular to the diode face, stray capacitance across the diode is minimized. Series inductance is minimized by making the spring as short as possible.
It is possible to mount the Sharpless wafer in the stripline geometry of a millimeter-wave integrated circuit. FIG. 1 of the drawings shows a directly adapted version of the prior art waveguide mounting in which the same type of diode chip is mounted in a millimeter-wave integrated circuit. The chip designated as 100 in FIG. 1 is cut small enough to meet the dimensional requirements of the stripline. It is mounted to the stripline by being set on its side and soldering the ohmic contact on its back surface 110 to a conductor 101 on a ceramic or glass substrate 105 in the millimeter-wave integrated circuit. Since the chip 100 only has an ohmic contact on its back surface, when set on its side as shown in FIG. 1, the solder which holds this chip to the stripline conductor assumes the shape of a fillet 102. A metallic spring 103 is mounted on a second conductor 104 and the pointed edge of the spring is positioned so as to contact a rectifying contact on chip 100.
The semiconductor diode chips of the prior art have a gold contact suitable for soldering only on the back surface of the chip because the sequence of processes required to make millimeter-wave Schottky barrier diodes requires that the ohmic contact be alloyed to the back of the semiconductor wafer before the diodes are formed on the front surface of the wafer. The ohmic contact must be made first otherwise the alloying step required to diffuse the contact into the semiconductor would destroy the diodes on the front surface as well. After the diodes are formed on the front surface the wafer is sawed into chips with the desired dimensions. It is not feasible technically or economically to cut the wafer first and process the chips individually. Two problems are confronted with the configurations shown in FIG. 1. First, the solder bond between the chip and stripline conductor must be a fillet joint because the sides of the chip are not able to be soldered. This joint is difficult to accomplish and is mechanically unreliable. Second, the small chip size required for millimeter-wave circuits is difficult to achieve since the epitaxial layer is susceptible to damage during the final process of sawing the wafer into chips. Such damage can extend about 0.001 to 0.002 inches from the edge of a cut and a minimum chip size is set by the need to have an undamaged area in the center of the chip.
The above-mentioned problems have been overcome by the use of an invention described in my copending application with Messrs. E. R. Carlson and A. A. Penzias filed simultaneously herewith. In accordance with this invention in my copending application, the back surface of a semiconductor wafer is first notched by a series of saw cuts spaced so as to section the wafer into individual areas corresponding to the chips but these cuts only extend approximately halfway into the wafer. After notching the back surface, the ohmic contact is established by depositing and alloying a metal contact on the back surface of the semiconductor wafer including the sides of the notches. Rectifying contact are then established on the front of the semiconductor wafer by a normal process. After establishing the diodes on the front surface the wafer is then broken along the fracture lines generally following the saw cuts produced during the notching process. The resulting notch-back chip can then be soldered into a millimeter-wave integrated circuit with solder adhering both to the bottom and along three edges of the chip yielding excellent mechanical and electrical results.
Although utilization of the invention in my copending application results in semiconductor diodes which are applicable to millimeter-wave thin-film circuits, the yield produced by this invention is not as high as desired since the breaking process is somewhat unpredictable and some of the chips produced have much different electrical and physical characteristics due to their irregular shape. Accordingly, it would be desirable to have a process for manufacturing semiconductor diodes of this type which would result in a higher yield of uniform semiconductor diodes.